In digital communication networks, mobile units must obtain information about the network and the serving base station before communication with the base station is possible. This process is often performed in three steps. The mobile unit must synchronize itself with the base station both in frequency and in time. Finally, the mobile unit obtains base station data from the signal transmitted by the base station.
In many communication systems, a mobile unit performs similar operations for handover purposes even when having a contact with a base station. A mobile unit may scan frequencies for signals of nearby base stations and store obtained information in case a handover must be performed quickly.
In many communication systems, the need for measurements has been taken into account in the air interface structure. The frame structure may provide the mobile units with time periods where different measurements may be performed.
In systems based on GSM (Global System for Mobile communication), the air interface structure is based on frames. Frames used to transmit traffic data comprise 8 time slots. Frames are organized as multiframes comprising 26 successive frames. Of these 26 frames, all other frames are used in transmitting traffic channels but the thirteenth frame, which is reserved for slow associated control channel, and the last frame, which is an idle frame. In the last frame, nothing is transmitted. Thus, a mobile unit may perform measurements during the last frame. The mobile unit may measure the signal strengths of neighboring base stations or search for new base stations.
In GSM based systems, the base stations transmit a frequency correaction channel (FCCH) so that a mobile unit may synchronize itself with the base station. The FCCH is transmitted on one frequency using a multiframe comprising 51 frames. The FCCH is transmitted in the first time slot of a frame five times during these 51 multiframes. Other control data is transmitted in the rest of the frames of the multiframe.
Typically, a mobile unit uses the following scheduling when searching for an FCCH. First, 0.5 time slots is required for switching from the current TX/RX frequency to the frequency where FCCH search is performed. One time slot is required for filling an FCCH correlator buffer, which is needed in order to obtain correct results when correlations are calculated. Eight time slots are spent for searching the FCCH. The search is performed by receiving a signal on the given frequency and correlating the signal with a known FCCH signal pattern. If a high correlation value is detected, it is likely that an FCCH has been found. Finally, 0.5 time slots are required for switching from the FCCH frequency to the current TX/RX frequency. Thus, the complete operation takes 10 time slots.
In most situations, the mobile units are able to perform the above operation without any problems. In normal operation, a mobile unit is transmitting and receiving on one time slot in both transmission directions (uplink and downlink). However, to support enhanced data communication with high bit rates, new multislot traffic classes have been proposed. In multislot operation, a mobile unit may utilize several time slots in a frame. For example, a mobile unit could receive using 7 time slots or the sum of time slots reserved for reception and transmission could be 6. These kind of multislot configurations may be utilized in packet switched data traffic or dual transfer mode situations where a mobile unit simultaneously has a circuit switched and a packet switched connection active.
A mobile unit having a high multislot connection is not able to perform the above described FCCH search during the idle frame. In a high multislot connection, the mobile unit may have a transmit or a receive time slot as the 7th slot of the frame. Thus, combined with the idle frame, the mobile unit may have only 9 time slots free for measurement purposes.
To circumvent the above problem, the current specifications allow a mobile unit to skip one RX or TX time slot in order to perform a full FCCH search. However, as other network elements are not aware when a mobile unit skips a time slot, the present method leads to a high number of retransmissions and reduced data throughput.